1. Field of Invention
This invention relates to a frequency error measuring apparatus, a radio apparatus and for example a digital cellular system for transmitting/receiving RF signals which include coded audio signals.
2. Description of the Related Art
Heretofore, in a digital cellular system, by using a coded audio signal, one channel is able to be used with a plurality of subscriber units at the same time by applying the method of time division multiple access system.
More specifically, when a radio wave arrives, this type of subscriber unit selects a channel having the strongest electromagnetic field after sequentially scanning, such as, 124 channels set in advance.
Then, the subscriber unit detects the control channel allocated to the area where this subscriber unit belongs and receives this control channel. The number of channels being able to be utilized in one cell is 124 in this system. The position of the control channel is not fixed in the 124 channels. Therefore, the apparatus of this systems scans sequentially all 124 channels, sorts these channels in the turn of strength of electromagnetic field, searches these channels to find frequency control channel (FCCH) signal in advance of sorted list, and determines whether channel is the control channel.
This control channel is used to establish a time slot and to transmit various information, and thus, the digital cellular system transmits information of the base station which receives this control channel from each subscriber unit and transmits, information of the adjacent base station, and furthermore, information to call out the subscriber unit.
Accordingly, the subscriber unit detects a frequency correction channel to be inserted to this control channel with the prescribed cycle and it adjusts processing timing based on this frequency control channel (FCCH), and simultaneously, roughly detects the timing of an existence of a necessary information.
The signal of the frequency control channel is the synchronizing signal which is allocated the bit pattern in order that the data with value "0" continues for the prescribed bit numbers when demodulated and in the digital cellular, this data is GMSK (Gaussian filtered Minimum Shift Keying) modulated and transmitted. Thus, as shown in FIG. 1, the signal of the frequency control channel signal can be expressed as composite waves of I signal and Q signal wherein their phase difference is 90.degree. and their signal levels change sinusoidally.
With this arrangement, when the signal reception result obtained by orthogonal detection at the subscriber unit is sampled with the correct timing, the resultant I,Q data successively circulate on the I axis and Q axis of complex planes with 90.degree. phase.
On the other hand, as shown in FIG. 2, if the frequency correction channel is received under the condition that the received frequency is not adjusted, the receiving data gradually shifts from I axis and Q axis corresponding to this frequency lag.
More specifically, this type of signal reception result can be expressed by vector. In the case of obtaining the signal reception result by sampling the signal reception result every successive bit (sampling is executed by 270 Kbps clock in the ASM system), if the frequency lag of this signal for the base station (i.e., clock delay in the subscriber unit) is expressed by .theta..sub.e (rad) and the sampled result of I,Q signal orthogonal detected in prescribed timing is expressed by vector S.sub.O (.alpha..sub.0 .beta..sub.0), and the signal reception result delayed by 4 samples is expressed by vector S.sub.4 (.alpha..sub.4, .beta..sub.4), the following equations can be obtained by using the amplitude/angle expression: EQU r.sub.0 exp (j.theta..sub.0)=.alpha..sub.0 +j.beta..sub.0 (1) EQU r.sub.4 exp (j.theta..sub.4)=.alpha..sub.4 +j.beta..sub.4 (2)
The frequency error can be expressed as follows: EQU .theta..sub.e =.theta..sub.4 -.theta..sub.0 (3)
Accordingly, the following equation can be obtained from equations (1) and (2): ##EQU1##
At this point, if the following relationship exists, EQU .theta..sub.e &lt;&lt;1 (5)
the following formula can be obtained: EQU sin .theta..sub.e =.theta..sub.e (6)
If the imaginary part of equation (4) is solved, the frequency error .theta..sub.e can be detected.
More specifically, the following equation can be obtained from equation (4): ##EQU2##
Thus, in the subscriber unit, the frequency error can be detected by executing calculation processing of equation (7) based on the signal reception result. In the actual subscriber unit, the frequency error .theta..sub.e will be detected with average processing in order to eliminate the effect of noise.
More specifically, since the amplitude of the signal reception result does not change much within one slot, the frequency error .theta..sub.e will be detected by executing the calculation processing of the following equation in the subscriber unit: ##EQU3##
In this case, since the frequency error .theta..sub.e comprises angle error of every 4 bit, the received signal circulates a greater number of times in the I,Q plane per second as defined in the following equation: ##EQU4## where R.sub.BIT is bit rate.
In this case, since bit rate is approximately 270.8 [Kbps] (13M bps/48) in the subscriber unit of the digital cellular system, the actual frequency error f.sub.e can be expressed as follows: ##EQU5##
Thus, the subscriber unit is capable of detecting frequency error .theta..sub.e and in practice, the processing procedure as shown in FIG. 3, is executed and the frequency deviation will be corrected.
More, specifically, the subscriber unit sequentially scans 124 channels which are set in advance. When the signal in the control channel allocated to the area where the subscriber unit belongs is received, the subscriber unit operates a step SP2 in FIG. 3 and it detects the correlation value sequentially between the input data comprising the reception result of the control channel and the prescribed reference signal.
Since the reference signal is defined as the identical with the signal of the frequency correction channel, the subscriber unit detects the timing of the signal of the frequency correction channel by detecting the rising of a correlation value.
Accordingly, when the subscriber unit detects the timing of the signal of the frequency correction channel, it stores the reception result of the frequency correction channel into the memory circuit by storing the I,Q data to be received with this timing in the prescribed memory circuit, and in the following step SP3 of FIG. 3, it executes the calculation process of equation (8) in utilizing I,Q data stored in this memory circuit.
With this arrangement, after detecting the frequency error .theta..sub.e the subscriber unit operates the step SP4 of FIG. 3 and corrects the clock frequency by correcting the oscillation frequency in the reference signal generation circuit based on the detection result of the frequency error .theta..sub.e. By operating the step SP5, it completes the processing procedure.
In practice, in the case that the subscriber unit is used in an environment with a bad receiving condition of the signal especially in mobile communications, a waveform distortion sometimes occurs in the received signal by noise and fading. Moreover, sometimes a carrier frequency of the received signal is displaced by the Doppler shift.
Therefore, in the conventional subscriber unit it has been difficult to detect the frequency of the frequency correction channel correctly due to kind of outside disturbance.
Especially, the conventional subscriber unit E.sub.b /N.sub.o =15[dB] is the limit of a capability of the frequency error detection according to the simulation result. (E.sub.b is a communicated energy by one bit. N.sub.o is a power density of noise. E.sub.b /N.sub.o is a common expression for expressing a state of a communication medium). It is clear that the frequency error cannot be detected practically if the noise level increases further.
However, in the practical application, a noise level increases more than this value. Sometimes it is necessary to receive the frequency correction channel repeatedly in the subscriber unit, and thus it takes time to start a communication.